An uninterruptable power-supply system (UPS) is configured to supply power continuously to a load. The UPS is stopped in some cases due to failure or maintenance of the UPS. To prepare for such cases, a bypass power supply is provided in parallel with the UPS.
A UPS generally includes an inverter. When the inverter fails, the bypass power supply provides power to a load. When the voltage of the bypass power supply is not in synchronization with the output voltage of the UPS, a large magnetizing inrush current disadvantageously flows into a load transformer.
When the bypass power supply provides power to the load, an output switch of the UPS is turned off. However, a certain amount of time is required to completely turn the output switch off. Therefore, there is a possibility that the magnetizing inrush current flows not only into the load transformer but also into an isolation transformer provided in the UPS.
Japanese Patent Laying-Open No. 8-256431 (PTD 1) discloses a static inrush current suppressing device for suppressing a transformer magnetizing inrush current. This device includes a thyrister connected to a primary side of a transformer, an instrument transformer connected to a primary side of the thyrister, a peak voltage detecting circuit connected to a secondary side of the instrument transformer, and a thyrister gate output circuit turning the thyrister on at a timing when the peak voltage detecting circuit detects a peak voltage. The transformer is magnetized by the peak voltage. Accordingly, the transformer magnetizing inrush current can be suppressed.